Chemical vapor deposition is a long-practiced technique in the field of semiconductor processing. Basically, chemical vapors, i.e., gases, are passed over heated substrates with which the gases react and deposit material on the wafer. Such material deposits may be polycrystalline (e.g., polycrystalline silicon), amorphous (SiO.sub.2, Si.sub.3 N.sub.4), homo-epitaxial (single crystal silicon deposited on single crystal silicon wafers), or hetero-epitaxial (single crystal silicon deposited upon substrates other than single crystal silicon, such as sapphire.)
In semiconductor processing, it is especially important that the material be deposited uniformly with uniform properties over the wafer. For instance, in integrated circuit technology, especially very large-scale integrated (VSLI) devices, the wafer is separated into chips, each of which is a separate integrated circuit. If a CVD process step does not deposit a uniform layer, each device on a wafer is different from another device and may have different operating characteristics. More likely, these variations generally lead to non-functioning or less than optionally functioning devices.
Wafers are processed in a series of steps. Any step which takes a long time slows manufacturing. Hence another goal of semiconductor manufacturing is to process as many wafers as possible at one time. In a CVD step, it is highly desirable for as many wafers as possible to be loaded into a CVD reactor at a time. Of course, the deposition uniformity requirements of the particular process must be met. Then the wafers can be sent on to the next step in the process.
There are two general classes of CVD reactors. These are the "hot wall" type or the "cold wall" type, depending upon whether the enclosing walls surrounding the deposition area in the reactor is at the same temperature as the deposition area (hot wall) or is colder than the deposition area (cold wall).
Two typical hot wall chemical vapor deposition reactors are shown in FIG. 1. FIG. 1A depicts a side view of a process chamber of a standard low pressure chemical vapor deposition (LPCVD) reactor. An LPCVD reactor with the wafers aligned with the gas flow is illustrated by a side view in FIG. 1B. The wafers in these hot wall reactors stand vertically in boats or carriers conserving space to permit a large number of wafers to be processed at once. The heating furnace is located outside the process chamber. Vacuum pumps and related equipment are not shown.
FIG. 2 depicts three different cold wall reactors. A vertical pancake reactor is shown in FIG. 2A, a horizontal slab reactor is shown in FIG. 2B and a cylinder reactor is shown in FIG. 2C. Typically ,the thin flat substrates of the semiconductor wafers are placed on a carrier or susceptor and heated to the desired temperature by induction, high intensity light radiation, electrical resistance, or a combination of such techniques. In all the geometries shown in FIG. 2, the substrates lay flat against the carrier and absorb energy from the carrier and from more remote sources by radiation, conduction and convection.
The reason for the bifurcation of CVD reactors lies in the fact that for processes in which the temperature and deposition thicknesses are comparatively low, the hot wall reactors are superior to the cold wall reactors in terms of uniformity and productivity. On the other hand, when the material must be deposited at comparatively high temperatures, the cold wall CVD reactors have the uniformity, quality and productivity advantage. It should be noted that the cold wall reactors have limited wafer capacity. Unlike the hot wall reactors into which wafers are arranged standing upright, the wafers must be placed flat against a carrier of limited area. This limits the total number of wafers which may be processed at one time.
The present invention overcomes the problem of cold wall reactors by allowing a large number of wafers to be processed at the same time without degradation of the quality of the deposited layer. A large number of wafers may be processed at comparatively high temperatures or low temperatures without compromising the uniformity of the deposited layer.
Additionally, reactors using plasma technology have a similar requirement as that of cold wall reactors. The wafers must be placed flat against a carrier or part of a carrier, of limited area. This limits the rate at which semiconductor wafers may be processed.
For reactors used in plasma-enhanced CVD, the present invention provides for a large number of wafers to be processed at once with uniformly deposited layers. The present invention also provides for a high volume plasma etching reactor in which material is removed from the wafers.